I bought some stainless steel pots at a yard/garage sale many years ago to use for dying. Before I picked these up, I used 3 pound coffee cans, which are good for 1 time use. The stainless pots are much better and the dye doesn't react with the stainless either so your colors are much "clearer and vibrant".
Your observations are correct on what are being sold as "spey feathers" by most suppliers. They are burnt goose. And you already know why I don't like burnt goose. Coche feathers (rooster tail) is what the original spey feather was (this information can be found in Knox's "Autumns on the Spey", and Kelson's "Salmon Fly") and they are far superior to burnt goose.
You are right about being able to get a fairly acceptable dun coloration with RIT black in either a weak dye bath or with cooler than normal temperature dye bath (as you indicate you have done); however, it has a hint of purple in the coloration that I find unacceptable. The Fly Dye dark dun does not have the purple overtone present with the RIT. With the Fly Dye, you get a good, bright, dark blue dun (the same color as Bleu-eared Pheasant or natural Heron) with ease. And the cost difference between a package or bottle of RIT and an once of Fly Dye is only a couple of dollars.
Keep in mind that RIT dye is not composed of the same dye stuffs as good quality acid dyes. RIT dyes are "shop" dyes that are formulated to work on fabrics, most of which do not react favorably with acid dyes because they are commprised of celulose or a nylon derivitive. Thus you have to use half a package to get a weak dye bath, or use more time in the dye bath, or more heat. And the colors obtained with RIT are inconsistent unless the time in the dye bath is kept to within 1 minute each time.
Quality acid dyes, like Fly Dye or Jacquard) are true acid dyes that were formulated for use with animal fibers, not celulose; thus, they require only 1/4 teaspoon of dye in a dye bath to dye several onces of material. And they are not sensitive to time in the dye bath. In fact, acid dyes should be dyed to exhaustion of the dye (meaning that there is little or no dye left in the bath-the water is either clear or nearly so) and lighter shades are simply a matter of using smaller amounts of dye in the dye bath. Acid dyes are also far more colorfast than RIT because the bond with the substrate molecules (animal fur or feather) extremely well.
After having used a good acid dye for the first time some 22 years ago, I have not used RIT since except for one special purpose: that is for dying white ringneck neck feathers yellow orange prior to overdying the tips scarlet for Indian Crow substitutes. This is done by using RIT golden yellow in a strong dye bath (1 complete package for 1/8 once of feathers) and left in the dye bath until the feathers have started to turn a decided orange hue. Then the feathers are removed from the dye bath, rinsed and after the water runs clear, dipped (tips only) in a strong scarlet dye bath for 10 to 30 seconds each feather, then the feathers are given a final rinse and placed on newspaper to dry.
The quality acid dyes do the job faster, have consistent colors, are easier to use, are colorfast, and can be mixed together for different shades without worrying about the resultant shade being "off" or "weird". RIT is really a very inferior dyestuff to use for dying feathers or fur.
Yes, I know that a very well-known tyer wrote and had publised a book (a book I have in my library only because I bought it sight unseen and I was expecting better information-in other words, I would not have bought the book if I had seen it first) on using RIT for dying fly tying material. I wish he wouldn't have because it has served to muddy the waters with having solid information on the dying process interspersed with his reliance on RIT. Even A.K mentions a preference for Veniard dyes (which are acid dyes but they suffer from most of the colors being a mixture of different dyes thus most of them are very time and amout sensitive), which he calls super powerful dyes because so little is needed to get great colors.
Although I do like and use Veniards hot orange, hot pink, and kingfisher blue-the best kingfisher blue available IMO); but that the Veniards dye is not easy for him to find. It is not available at the local grocery, K-Mart, or Wal-Mart store like RIT.